As is known, an optical fiber is designed to convey light waves, and comprises an optical portion or "bare optical fiber", made up of an optical core surrounded by optical cladding, generally covered with at least one protective layer, and optionally with an identification layer which is usually colored. Advantageously, such optical fibers can be grouped together into optical fiber ribbons, in which case the optical fibers are disposed side-by-side, parallel to one another and substantially in the same plane, and they are embedded in a common protective matrix. Such an optical fiber ribbon is generally used for manufacturing a high-density optical cable, e.g. a telecommunications cable, for conveying information from an information provider to a user situated at a point remote from the provider. At the various users, the optical fibers are distributed in various bundles of optical fibers, which means that it is necessary to separate them without damaging them, i.e. without degrading the transmission characteristics of the optical fibers. That is why it is advantageous to use a multi-ribbon optical fiber structure having two substantially flat faces and containing mutually-parallel optical fibers disposed side-by-side substantially in the same plane which then defines the two substantially flat faces of said multi-ribbon structure, and embedded in a common protective matrix, said multi-ribbon structure being separable into at least two optical fiber ribbons.
When using a multi-ribbon optical fiber structure, it would be unnecessarily costly to require additional equipment in situ for separating the multi-ribbon structure into at least two optical fiber ribbons. That is why multi-ribbon structures have been proposed in which scores are provided along at least one longitudinal score line so that they can be subsequently split up into ribbons along the score lines, the scores being formed on coating the optical fibers with at least one matrix-forming covering layer of resin, by molding the resin and then thermosetting it. Thus, U.S. Pat. No. 5,442,722 mentions a first piece of prior art in which scoring is obtained directly by molding the matrix in a manner such as to form a "necked-down" area in it (see FIG. 1 of that patent, showing the prior art in question). That method does not make it possible to obtain ribbons having dimensions that are sufficiently accurate. U.S. Pat. No. 5,442,772 itself relates to technology for opening up the ribbon by means of one or more "zip cords" or "ripcords" inserted in the matrix. Thermosetting is achieved by irradiation, e.g. by curing or "crosslinking" a resin under the effect of ultraviolet (U.V.) radiation, the resin being, for example, based on urethane acrylate. The ultraviolet radiation serves to provide the energy necessary for the curing reaction. Similarly, Patent Application EP-A1-0 647 866 describes a technology based on opening up the ribbon by means of at least one ripcord inserted in a matrix, opening optionally being facilitated by forming scores in the ribbon.
Unfortunately, such technologies have, in practice, been disappointing. Firstly, it is difficult to position the molding apparatus on the multi-ribbon optical fiber structure with accuracy that is high enough to prevent the optical fibers from being damaged. Secondly, a score formed by molding in resin that is still fluid tends to fill with resin soon after molding, especially when the optical fibers advance at high speeds through the apparatus for manufacturing the multi-ribbon structure. Finally, inserted ripcord technology is complicated to implement because the ripcord must be positioned very accurately in the resin matrix, and above all, that technology does not make it possible to separate a multi-ribbon structure elsewhere than at its ends, because it is not possible to access the ripcord except at the ends of the multi-ribbon structure.
Consideration has also been given to scoring the covering as described in the second piece of prior art mentioned in U.S. Pat. No. 5,442,772. Thus, Japanese Patent Application JP-A-1 138 516 proposes a method of manufacturing a multi-ribbon optical fiber structure that has two substantially flat faces, and that can be separated into at least two optical fiber ribbons. That method involves manufacturing two initial ribbons, then partially pre-thermosetting portions of the two initial ribbons, the portions being pre-thermoset together to form the optionally necked-down central portion of the multi-ribbon structure, and being designed to be separated subsequently, and finally fully thermosetting the assembly comprising the two adjoining ribbons so as to form the final multi-ribbon structure, followed by scoring one of the two faces of the multi-ribbon structure with intermittent scoring apparatus including scoring means of the mechanical type (cutting disk with associated backing disk) or of the laser type (laser beam emitter), the scoring being performed in the central portion of the multi-ribbon structure so as to form discontinuous scores spaced apart at a fixed and preferably constant pitch (in the form of "dashes").
The problem that arises in Japanese Patent Application JP-A-1 138 516 is that of manufacturing a multi-ribbon structure such that the two initial ribbons making it up are accurately aligned. The two initial ribbons might slip relative to each other while the resin serving to form the matrix of the multi-ribbon structure is still fluid, i.e. not cured or partially cured only, or while it is solid, i.e. almost totally cured. Such slippage, even if it is only slight, is highly detrimental to the resulting multi-ribbon structure, and is difficult to control when manufacturing industrially. This applies even more so when the optical fibers advance at high speeds through the apparatus for manufacturing the multi-ribbon structure. Furthermore, the scoring apparatus such as it is implemented in said patent application requires the two optical fibers in the central portion of the multi-ribbon structure, where the intermittent scoring takes place, to be spaced apart by a distance greater than the distance between adjacent optical fibers in the ribbons. Thus, the spacing between two adjacent optical fibers making up the multi-ribbon structure must be constant except between the two optical fibers side-by-side in the central portion of the multi-ribbon structure, between which fibers the spacing must be greater.